Edit from field in personality and external account

Hi, if I'm running zimbra 7 and I wish to be able to edit the "from" field in a personality.
I have a test user and if I try to add a personality, I can only select my same email address from the list.
I know it's possible to have a text field instead of the drop down list, but I don't remember how to enable that.
I've been googeling withouth success, probably using the wrong key words.